Covering a land drain with lawn

Hello all
First post on the site so hope i don't give away what an amateur I am!
I've recently cleared an area of rubble, hardcore and clay soil where there used to be a poorly laid patio so that it is at the same level as the rest of the lawn with the intention of turfing the area so that we have a bigger lawn for the boys to play on.
At the far end of the cleared area is what I think is called a land drain. It's a rectanguar manhole cover over a brick "chimney" (like a short square well?) down to about 10 feet below, where there are three large bore pipes that come from the rest of the land generally. There is no water at the bottom, so I presume that this was put in when the house was built (70s) just to aid drainage of the land around?
Rather thatn turf around the manhole cover which is (a) unattractive and (b) not ideal to have in an area where three small boys and there friends will be charging around!
Since we've lived here for a few years now, including some very wet ones and never had reason to open the cover (didn't eve know what it was for until this little project began), I'm going to take down the top couple of layers of bricks and replace the manhole cover further down so that I can then put soil over the top and turf it over.
My questions then to the forum are these...
Firstly any tips on how far down I need to go, i.e. How much soil will the grass need, not really for drainage as it,s only a couple of feet by one foot, but more relevant I think so that the area holds sufficient moisture so that it doesn't all crack up in the summer.
Secondly what's the best way of covering the top of the ground drain so that it remains secure and doesn't give way after a couple of years of being buried with water etc...
Look forward to all the helpful replies!
Thanks
Jem

Definately agree with Sue here. Would leave well alone and certainly not take any bricks out of the main 'collar'. You just never know when access might be needed in the future. Is there nothing on the original house plans to indicate what it is actually for? Is anyone nearby linked to a septic tank? It could also be part of a 'mains' sewerage system! I'm certainly no expert at all, but I would definately make enquiries into what it actually is before starting any potentially damaging work at all.

It sounds just like the manhole I have in my front garden, which is part of the sewerage system. You could try flushing the loo and look for and listen to any water running through the pipes. I believe it is okay to change the level of the manhole cover by removing or adding bricks and replacing the lid at the new level, but it should never be covered up. I think a builder is the best person to do this, if you are not sure. I keep a large planter on my manhole cover, just to hide it, but it can easily be removed for access. I once had a problem with my toilet, which had to be investigated. The plumber accessed the manhole to see if there was a blockage in the pipes. There was no water visible until the loo was flushed and then the water rushed straight through on its journey to the sewer. He later accessed the sewer itself and found the problem, which the water board had to sort out.

Hi Jem,
I actually did cover mine (in another garden - some years ago) and turf over it. No structural alterations were made and the corners were discreetly marked for access, which was never required! I didn't give sufficient depth of soil and therefore, during hot summers, the grass always dried out. Having said that, it always recovered.
Hope this helps,
Pam Thompson
www.peartreecottage.me

Many good and valid points already suggested. Have you taken the trouble to check the original plans for the house including the drain system. Make certain that the manhole/drainage route is no longer in use. Although most people probably would bury the cover with soil etc. There might be some local governement ruling concerning drains etc. I believe someone already has mentioned the dry summer etc where eventually the tuf displayed serious signs of dryness. Basic equation. ? amount of soil to cover the drain vs, raising total area....provided you wish to have a flat level lawn.